fix(html): keyword-in-heading badge + brackets in wikilink descriptions

Two export bugs reported against v0.4.0:

1. A heading whose text is a keyword (`== TODO ==`) rendered as an inline
   `<span class="todo">` badge with an *empty* id, instead of a styled
   header. Root causes: `inline_text` dropped Keyword nodes (so the heading
   id/anchor came out empty, or `My TODO list` → `My  list`), and the
   keyword span was emitted inside the heading. Fix: `inline_text` now
   includes the keyword's literal text (via new `Keyword::label()`), and
   heading content renders keywords as plain text (flatten_keywords) so a
   keyword title looks like a header. Keyword badges still render in body
   text as before.

2. A `]` inside a wikilink description (e.g. `[[page|Task [B-1]]]`) closed
   the link at the first `]]`, truncating the description and leaking a
   stray `]`. The close-scan is now bracket-aware (find_wikilink_close):
   inner `[ ]` pairs are balanced so the link closes at the correct `]]`.

Added regression tests for both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/nuwiki-core/tests/html_renderer.rs

@@ -77,6 +77,43 @@ fn centered_heading_gets_centered_class() {
     assert!(out.contains("<h1 class=\"centered\" id=\"T\">T</h1>"));
 }
 
+#[test]
+fn heading_keyword_renders_as_plain_text_not_badge() {
+    // `== TODO ==` is a section title, not an inline keyword: it must render as
+    // a normal header (with a correct, non-empty id), not a `<span class="todo">`
+    // badge. Regression for the exported header losing its header styling + id.
+    let out = render("== TODO ==\n");
+    assert!(
+        out.contains(
+            "<div id=\"TODO\"><h2 id=\"TODO\" class=\"header\">\
+             <a href=\"#TODO\">TODO</a></h2></div>"
+        ),
+        "got: {out}"
+    );
+    assert!(!out.contains("class=\"todo\""), "no keyword badge: {out}");
+
+    // A keyword mid-title keeps its text in the id (no dropped word / double
+    // space) and still renders plainly.
+    let out2 = render("= My TODO list =\n");
+    assert!(out2.contains("id=\"My TODO list\""), "got: {out2}");
+    assert!(!out2.contains("class=\"todo\""), "got: {out2}");
+}
+
+#[test]
+fn wikilink_description_keeps_inner_brackets() {
+    // A `]` inside a wikilink description (e.g. a `[TICKET-1]`) must not close
+    // the link early. Regression for descriptions with brackets being mangled.
+    let out = render("[[page|Task [B-1]]]\n");
+    assert!(
+        out.contains("<a href=\"page.html\">Task [B-1]</a>"),
+        "got: {out}"
+    );
+    // Two bracketed links on one line both resolve.
+    let out2 = render("[[a|X [1]]] and [[b|Y [2]]]\n");
+    assert!(out2.contains("<a href=\"a.html\">X [1]</a>"), "got: {out2}");
+    assert!(out2.contains("<a href=\"b.html\">Y [2]</a>"), "got: {out2}");
+}
+
 #[test]
 fn paragraph_wraps_inline_content() {
     let out = render("Hello world\n");